1. The Field of the Invention
The present invention is related generally to thin film optical coatings for use in producing security articles. More specifically, the present invention is related to the production of diffractive surfaces such as holograms or gratings having color shifting or optically variable backgrounds which can be used as security articles in a variety of applications.
2. The Relevant Technology
Color shifting pigments and colorants have been used in numerous applications, ranging from automobile paints to anti-counterfeiting inks for security documents and currency. Such pigments and colorants exhibit the property of changing color upon variation of the angle of incident light, or as the viewing angle of the observer is shifted. The primary method used to achieve such color shifting colorants is to disperse small flakes, which are typically composed of multiple layers of thin films having particular optical characteristics, throughout a medium such as paint or ink that may then be subsequently applied to the surface of an object.
Diffraction patterns and embossments, and the related field of holographs, have begun to find wide-ranging practical applications due to their aesthetic and utilitarian visual effects. One very desirable decorative effect is the iridescent visual effect created by a diffraction grating. This striking visual effect occurs when ambient light is diffracted into its color components by reflection from the diffraction grating. In general, diffraction gratings are essentially repetitive structures made of lines or grooves in a material to form a peak and trough structure. Desired optical effects within the visible spectrum occur when diffraction gratings have regularly spaced grooves in the range of hundreds to thousands of lines per millimeter on a reflective surface.
Diffraction grating technology has been employed in the formation of two-dimensional holographic patterns which create the illusion of a three-dimensional image to an observer. Furthermore, the use of holographic images on various objects to discourage counterfeiting has found widespread application.
There currently exist several applications for surfaces embossed with holographic patterns which range from decorative items, such as gift wrap, to security documents, such as bank notes and credit cards. Two-dimensional holograms typically utilize diffraction patterns which have been formed on a plastic surface. In some cases, a holographic image which has been embossed on such a surface can be visible without further processing; however, it is generally necessary, in order to achieve maximum optical effects, to place a reflective layer, typically a thin metal layer such as aluminum, onto the embossed surface. The reflective layer substantially increases the visibility of the diffraction pattern embossment.
Unfortunately, there exists a substantial incentive for counterfeiters to reproduce the holograms which are frequently used in credit cards, bank notes, and the like. One of the methods used to reproduce holograms is to scan a laser beam across the embossed surface and optically record the reflected beam on a layer of a material such as a photopolymerizable polymer. The original pattern can subsequently be reproduced as a counterfeit. Another method is to remove the protective covering material from the embossed metal surface by ion etching, and then when the embossed metal surface is exposed, a layer of metal such as silver (or any other easily releasable layer) can be deposited. This is followed by deposition of a layer of nickel, which is subsequently released to form a counterfeiting embossing shim.
Due to the level of sophistication of counterfeiting methods, it has become necessary to develop more advanced security measures. One approach, as disclosed in U.S. Pat. Nos. 5,629,068 and 5,549,774 to Miekka et al., is the application of inks, such as metallic flake inks, metallic effect inks, or inks with pigments formed of optical stacks, upon the embossed surface in lieu of a thin metal layer. In another approach, disclosed in U.S. Pat. Nos. 5,624,076 and 5,672,410 also to Miekka et al., embossed metal particles or optical stack flakes are used to produce a holographic image pattern.
Another problem with the holographic images as described above is that they require direct specular illumination in order to be visualized. This means that for best viewing results, the illuminating light must be incident at the same angle as the viewing angle. Therefore, diffuse light sources, such as ordinary room lights or viewing by an overcast sky, when used to illuminate the holographic image, do not reveal much of the visual information contained in the hologram, and what is typically seen is only a silver colored reflection from the embossed surface.
It would therefore be of substantial advantage to develop improved security products to provide enhanced viewing qualities in ordinary room light and which are usable in various security applications to make counterfeiting more difficult.